Hydride complexes classical

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

Let us now consider a few non-classical hydride complexes for which molecular structures are available. 

It is useful to note that the simple substitution of the counteranion to form [Co(triphos)(H2)][BPh4] produces a classical hydride complex. This is a further demonstration of the lability which characterizes the boundary between classical and non-classical hydrides. 

Regio- and stereo-selective dimerization of alk-l-ynes catalysed by classical and non-classical hydride complexes of Ru(II) and Os(II) stabilized by the tripodal polydentate ligand (Pl PCH CH P has been reported to produce the corresponding (Z)-1,4-disubstituted butenynes. Irrespective of the nature of the hydride ligand (classical or non-classical), vinylidene complexes appear to be the immediate precursors to the C—C bond-forming step.67... 

Although both the alkyl complex [Co(nor)4] (Figure 15) and the hydride complex [Co(C5Me5)(Si(H)Ph2)2(H)2] (Figure 16) offer well-characterized examples of organometallic Co(IV) and Co(V) complexes, the situation with classical coordination complexes is much less clear. 

Certain classical coordination complexes (see Coordination Complexes) of iron (e.g. Prussian blue) will be dealt with in other articles (see Iron Inorganic Coordination Chemistry and Cyanide Complexes of the Transition Metals), as will much of the chemistries of iron carbonyls (see Metal Carbonyls) and iron hydrides (see Hydrides) (see Carbonyl Complexes of the Transition Metals Transition Metal Carbonyls Infrared Spectra, and Hydride Complexes of the Transition Metals). The use of organoiron complexes as catalysts (see Catalysis) in organic transformations will be mentioned but will primarily be covered elsewhere (see Asymmetric Synthesis by Homogeneous Catalysis, and Organic Synthesis using Transition Metal Carbonyl Complexes). 

Among the classical hydride complexes there are not only monohydrides and dihydrides, but also polyhydrides. Well-known trihydrides are ( -C5H5)2NbH3... 

The nonclassical hydride complex, W(i7 -H2)(CO)3(PCy3)2, exists in solution in equilibrium with its classical tautomer W(H)2(CO)3(PCy3)2 . Similar equilibria have been observed for di- and polyhydrides -. Nonclassical hydride ligands must certainly exist in equilibrium with some catalytically active classical hydride complexes. As intermediates to more reactive classical hydrides, non-classical hydrides would simply tie up available catalyst. 

In addition, the deprotonation process allows additional stoichiometric variations depending on the stability of the protonated hydride complex MH2+, which may prefer either the classical or the nonclassical tautomeric form. In the latter case, the product may be unstable, depending on the reaction conditions, toward replacement of H2 by a solvent molecule, leading to the possible alternative stoichiometries of equations 26-28. 

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